Tailoring amorphous boron nitride for high-performance two-dimensional electronics

Two-dimensional (2D) materials have garnered significant attention in recent years due to their atomically thin structure and unique electronic and optoelectronic properties. To harness their full potential for applications in next-generation electronics and photonics, precise control over the dielectric environment surrounding the 2D material is critical. The lack of nucleation sites on 2D surfaces to form thin, uniform dielectric layers often leads to interfacial defects that degrade the device performance, posing a major roadblock in the realization of 2D-based devices. Here, we demonstrate a wafer-scale, low-temperature process (<250 °C) using atomic layer deposition (ALD) for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. ALD deposition temperatures between 125 and 250 °C result in stoichiometric films with high oxidative stability, yielding a dielectric strength of 8.2 MV/cm. Utilizing a seed-free ALD approach, we form uniform aBN dielectric layers on 2D surfaces and fabricate multiple quantum well structures of aBN/MoS2 and aBN-encapsulated double-gated monolayer (ML) MoS2 field-effect transistors to evaluate the impact of aBN dielectric environment on MoS2 optoelectronic and electronic properties. Our work in scalable aBN dielectric integration paves a way towards realizing the theoretical performance of 2D materials for next-generation electronics.

(Q1) Even if an aBN sample grown at 65 degrees undergoes subsequent post-treatment at 200 degrees in the chamber, will it have oxygen contents like the as-grown sample without heat treatment?(Q2) If the aBN sample grown at a low temperature of 65 degrees reacts with atmospheric oxygen, it is thought that the reaction will be concentrated on the surface.If you grow a thick a-BN sample and analyze the atomic concentration in the depth direction, will there be a change?Or is there a difference depending on the thickness of the aBN thin film in the oxygen level measured by the surface?(Q3) Why is the oxygen content of the hybrid a-BN film lower at 3% than the 4% of the thin film grown at 200 degrees?
A trench pattern with 10 x 10 um2 area and 500 nm depth was used to demonstrate conformal growth behavior.However, in order to more clearly assert the advantages of ALD, the top/side/bottom situation must be shown in a trench pattern with a higher aspect ratio.
(Q4) Have you tested in a high aspect ratio structure?
The authors claim that hBN can form a clean vdW interface with the 2D semiconducting channel.It can substantially reduce charge carrier scattering due to surface roughness and charged impurities, also leads to reduced remote phonon scattering since the high energy surface optical phonon modes of hBN do not couple to the low energy modes in 2D semiconductors.However, a-BN has structural/physical properties that are significantly different from h-BN.It has no vdW interface and even dielectric constant is not high as much as the others, such as HfO2 and Al2O3.
(Q5) Nevertheless, is there any basis for judging that it is useful as a dielectric in 2D electronic devices?(Q6) Two step approach at different tempeature is applied to deposit aBN on MoS2.What happens if 250 degree aBN is deposited directly on the MoS2 surface without low-temperature aBN deposition?(Q7) What is the minimum thickness of aBN before HfO2 deposition for MoS2 device fabrication?Reviewer #2 (Remarks to the Author): Chen et al. have reported on the growth of Amorphous Boron Nitride (aBN), highlighting its potential applications in post-Moore era electronics.It is commendable to approach material growth with a specific focus on electronic devices and circuits.However, despite this perspective being reiterated in the title, abstract, and introduction, the paper lacks a clear demonstration of how this material growth directly contributes to the enhancement of device parameters.For highperformance digital electronic devices, particularly ultra-scaled short-channel devices in advanced technical nodes below 10 nm, it is crucial.Yet, the devices examined in this study still operate within the diffusive regime.Furthermore, 'high-performance' should also imply superior transistor characteristics, such as high on-state current and fast switching with low power consumption (intrinsic gate delay and energy-delay product).Regrettably, these aspects are overlooked in the paper, weakening the claim of 'high-performance.' Additionally, I have several concerns which I outline below: 1. What is the defect density in aBN?Can the authors provide spectroscopy data to quantify the defect condition and explain how these defects impact transistor behavior?Also, can the interfacial trap state density and border trap density be quantified?2. The gate control efficiency is influenced by the thickness of the aBN dielectric.Can the authors compare the efficiency of single-layer or bilayer aBN with high-k dielectric in terms of gate efficiency improvement?3. Regarding the extraction of the SS values, its significance diminishes as the current approaches the off-state.As shown in Figure 4d, at an off-state current of 100 nA/um, the SS value is 200 mV/dec, far from the "Near ideal transport characteristics" claimed for ALD aBN-encapsulated monolayer MoS2 FETs. 4. Could the authors elaborate on achieving "trap charges from the bottom dielectric layer, not exceeding 10^12 cm^−2"? 5. Providing additional experimental evidence for the abnormal k values of the aBN grown at 65 degrees Celsius would be beneficial.
Reviewer #3 (Remarks to the Author): Cindy Y. Chen et al. demonstrated a wafer-scale, low-temperature ALD process of aBN for integration with 2D material-based devices.The authors controlled the chemical composition of aBN by changing the deposition temperature from 65 to 250 oC.They fabricated not only the quantum well structure (barrier/semiconductor/barrier) but double-gated FET structures using aBN interfacial layered HfO2 gate dielectric.As a result, their device performances considerably improved.Interestingly, the intrinsic dielectric properties of ALD aBN do not show low dielectric constant.They varied from 4.3 to 8.6 at 100 kHz, depending on the deposition temperature.This result conflicts with previously reported aBN literature [REF 37-40 in original manuscript].Therefore, it is necessary to define the differences compared to reported aBN structures.In this aspect, I believe that this work may be considered for publication after correcting somewhat misleading statements.1.Why does aBN show a relatively high dielectric constant (k)?They have higher k than crystalline BN (~3.2).For example, if their deposition temperature exceeds 300 oC, does k value decrease?2. In Figure 1b, they measured the Raman spectrum of aBN on a Si substrate to confirm a noncrystalline BN structure.This reviewer recommends adding the aBN Raman spectrum measured on the SiO2 substrate because detecting the E2g signal of thin crystalline BN is difficult on the Si substrate.Furthermore, many characterizations of aBN are conducted on SiO2 in this study.The oxide layer can affect the crystallinity of BN during deposition.3. To clarify the thickness of the aBN film, the authors have to show the height profiles of their aBN films as a function of deposition temperature using atomic force microscopy.4. In the aBN/MoS2 quantum well stack structure, what is the difference between hybrid ALD (65 oC and 250 oC) and 250 oC ALD for a device? 5.In Figure 3e, the EDS map of N is unclear to readers.It should be revised to a clear image.
The current manuscript delineates the ALD-based a-BN growth techniques and its applicability in the field of 2D electronics.The structural/chemical properties of aBN materials in relation to growth temperature have been systematically analyzed, and strategies for their implementation in 2D electronics have been clearly elucidated.In light of the significant efforts being expended in the pursuit of gate dielectric and interface materials for 2D electronics, it is envisaged that the outcomes of this study will draw the attention of the relevant community.Nevertheless, for this manuscript to be deemed suitable for publication in Nature Communications, I recommend providing further clarification and supplementation on the following queries.To determine the cause of the influx of a significant amount of oxygen at low temperatures, the authors produced and analyzed a hybrid a-BN film that was grown sequentially at low and high temperatures.However, there are still unclear elements in claiming that the a-BN thin film grown at 65 degrees does not deform during the subsequent a-BN thin film growth process at 200 degrees.(Q2) If the aBN sample grown at a low temperature of 65 degrees reacts with atmospheric oxygen, it is thought that the reaction will be concentrated on the surface.If you grow a thick a-BN sample and analyze the atomic concentration in the depth direction, will there be a change?Or is there a difference depending on the thickness of the aBN thin film in the oxygen level measured by the surface?
As the data suggests oxidation of unreacted B-Cl bonds upon air exposure, oxidation is thought to occur initially from the surface to the bulk film.Based on this finding, we expect immediately upon air exposure, a thicker 65 o C aBN film should initially have B-O components concentrated on the surface, and extend towards the bulk as the time exposed to ambient increases.
(Q3) Why is the oxygen content of the hybrid a-BN film lower at 3% than the 4% of the thin film grown at 200 degrees?A trench pattern with 10x10 um 2 area and 500 nm depth was used to demonstrate conformal growth behavior.However, in order to more clearly assert the advantages of ALD, the top/side/bottom situation must be shown in a trench pattern with a higher aspect ratio.
(Q4) Have you tested in a high aspect ratio structure?
We thank the reviewer for the suggestion.We have added the top/side/bottom image in the SI (Supplementary Figure 2).As we have not tested ALD on a higher aspect ratio structure, we have modified the text (Main Text, page 3 and 5) to emphasize conformality and uniform step coverage on structured surfaces rather than high aspect ratio structures.
The authors claim that hBN can form a clean vdW interface with the 2D semiconducting channel.It can substantially reduce charge carrier scattering due to surface roughness and charged impurities, also leads to reduced remote phonon scattering since the high energy surface optical phonon modes of hBN do not couple to the low energy modes in 2D semiconductors.However, a-BN has structural/physical properties that are significantly different from h-BN.It has no vdW interface and even dielectric constant is not high as much as the others, such as HfO2 and Al2O3.
(Q5) Nevertheless, is there any basis for judging that it is useful as a dielectric in 2D electronic devices?
From our experimental results, aBN is a better seeding layer compared with other oxide-based seeding layer, such as AlOx and TaOx 1,2 .Despite a strong (desired) n-doping effect that is a result of the use of oxide seeding layers, a substantial degradation of the inverse subthreshold slope and off-state current is detrimental for any device performance.In our work, when using aBN as an interfacial layer to form the top gate, both high on-state currents and good off-state performance are achieved simultaneously.
Previous works indicate that FET of graphene on aBN shows performance improvement in FET mobility and carrier homogeneity and reduced extrinsic doping in graphene compared to graphene FET with SiO2only dielectric.4] In addition, aBN was proved suitable as a passivation and heat passivation layer on top of TMDs 5 or between TMDs and oxide dielectric (where aBN can reduce interfacial thermal resistance) 6 that helps improve TMD's FET performance.While its relatively low k doesn't help push for a smaller device EOT, Ref. [3][4][5][6] and our ALD-grown aBN show that aBN can be useful in 2D electronic devices with different approaches.] As more people are looking into the back-end integration and low-temperature synthesis for 2D semiconductors, aBN on TMDs like what we demonstrate in this manuscript will play an important role down the roads.
Another technology-related point in favor of aBN is that future 2D-based nano-sheet FETs are expected to be used in conjunction with a gate-all-around (GAA) geometry.In this context, an ALD-grown amorphous BN will be beneficial as seeding layer for ML MoS2 GAA FETs, since it can be uniformly deposited everywhere on the MoS2 layer.Directional e-beam deposition of an oxide-based seeding layer is not an option in this case due to the inability to cover the "back side" of the nano-sheet channel.
(Q6) Two step approach at different temperature is applied to deposit aBN on MoS2.What happens if 250 degree aBN is deposited directly on the MoS2 surface without low-temperature aBN deposition?
We show that a one-step aBN deposition at 250 o C on MoS2 yields a discontinuous aBN film with nucleation occurring preferentially along the MoS2 grain boundaries (Supplementary Figure 5a).This is further quantified and supported by XPS analysis, in which aBN film deposited at 250 o C without the lowtemperature step exhibits a B/S ratio ~0 (Supplementary Figure 6c).Reviewer #2 (Remarks to the Author): Chen et al. have reported on the growth of Amorphous Boron Nitride (aBN), highlighting its potential applications in post-Moore era electronics.It is commendable to approach material growth with a specific focus on electronic devices and circuits.However, despite this perspective being reiterated in the title, abstract, and introduction, the paper lacks a clear demonstration of how this material growth directly contributes to the enhancement of device parameters.For high-performance digital electronic devices, particularly ultra-scaled short-channel devices in advanced technical nodes below 10 nm, it is crucial.Yet, the devices examined in this study still operate within the diffusive regime.Furthermore, 'high-performance' should also imply superior transistor characteristics, such as high on-state current and fast switching with low power consumption (intrinsic gate delay and energy-delay product).Regrettably, these aspects are overlooked in the paper, weakening the claim of 'high-performance'.
We do agree with the reviewer in general that ultra-scaled short-channel devices with channel lengths below 50 nm are important to ultimately achieve high on-current levels.However, as shown in Figure R2.1 below, the contact resistance in our case is still in the thousands of ohm•µm range, far from the desired quantum limit of contact resistance.This implies that even if the device channel is scaled towards 10 nm, one does not expect any improvement in device performance, which is the reason that such a study has not been performed in this manuscript which focuses on the gate stack and not contacts.Instead, our focus has been on demonstrating superior off-state behavior, including good inverse subthreshold slopes and very small hysteresis behavior below a threshold.Our findings indicate that good electrostatic gate control has been achieved in our gate dielectric stack that involves aBN.As mentioned above, while higher on-current levels have indeed been achieved in double gate MoS2 FETs, those devices showed a severely degraded, unacceptable off-state behavior.
We also note that ALD-grown aBN offers several process advantages over crystalline hBN for the continuous improvement of dielectrics in 2D electronics.Compared to the "transfer-required" CVD-grown hBN (or exfoliated hBN crystals), ALD-aBN is performed at significantly lower process temperatures (< 250 o C) and on a wafer-scale, thus providing a better approach towards future 2D electronic integration schemes.Another technology-related point in favor of aBN is that future 2D-based nano-sheet FETs are expected to be used in conjunction with a gate-all-around (GAA) geometry.In this context, an ALD-grown amorphous BN will be beneficial as seeding layer for ML MoS2 GAA FETs, since it can be uniformly deposited everywhere on the MoS2 layer.Directional e-beam deposition of an oxide-based seeding layer is not an option in this case due to the inability to cover the "back side" of the nanosheet channel.
Additionally, I have several concerns which I outline below: 1. What is the defect density in aBN?Can the authors provide spectroscopy data to quantify the defect condition and explain how these defects impact transistor behavior?Also, can the interfacial trap state density and border trap density be quantified?
One way of mapping out the defect density in aBN is the usage of STM spectroscopy.While we did not perform this type of study, our experimental transport data provide quantitative information about the interface trap density in our gate stack structures.From the inverse subthreshold slope SS in the device offstate, we extract an interfacial trap state density of 1.098*10 12 cm -2 .This value is extracted by noting that in the deep off-state that is dominated by thermal emission over the conduction band edge of MoS2, SS = 2.3*kT/q*n, where n=1+(CD+Cit)/Cox.Noting that CD is zero for a fully depleted device as ML MoS2, n= 1+Cit/Cox in our case.As shown in Figure 4h in the main text of the manuscript, SS is around or smaller than 80 mV/dec, which implies a Cit of 0.33*Cox.
To carefully extract Cox = CMOS from the back-gated ML MoS2 FETs, we measured a MOS capacitor (CAP) of ML MoS2 on HfO2/aBN dielectric (Figure R2.2), and determined an oxide capacitance of 0.523 uC/cm 2 , which means that Cit is around 0.174 µF/cm 2 , which equals to 1.086×10 12 eV -1 cm -2 V -1 The main goal of our ALD process is to produce pure amorphous BNwithout any nanostructures at the atomic scale.Our TEM data in the paper provide clear evidence about the amorphous structure.Techniques such as STM/STS and Raman and CL spectroscopy commonly seen on qualifying hBN are not useful for our aBN.Our XPS analysis indicates that the B to N ratio is 0.93 ± 0.02, which suggests our aBN films might be slightly boron deficient.Thus, this cation deficiency may contribute to the p-doping effect that prevents VTH shift toward more negative due to charge interaction between amorphous oxide dielectric (HfO2 in this study) and MoS2 FET.
2. The gate control efficiency is influenced by the thickness of the aBN dielectric.Can the authors compare the efficiency of single-layer or bilayer aBN with high-k dielectric in terms of gate efficiency improvement?
As the aBN films are amorphous, they exhibit no crystalline order and are not layered structures.Here, we consider two aBN thickness values: 3.6 and 1.8 nm.The metal-insulator-metal capacitance -CMIMfor our gate stack (3.6nm aBN+5nm HfO2) has been experimentally determined to be 0.695 µF/cm 2 .Due to the existence of an "air gap" between the dielectric and the ML MoS2 3,4 , the measured value for CMOS is found to be always smaller than CMIM.For the same 3.6 nm aBN+5 nm HfO2 gate stack, we measured CMOS to be around 0.57 µF/cm 2 .
Once the dielectric film thickness for aBN is scaled down from 3.6 to 1.8 nm, the CMIM of 1.8nm aBN + 5nm HfO2 is expected to be 1.07 µF/cm 2 .This means from Figure R2.3 that CMOS is expected to be around 0.8 µF/cm 2 , 1.4 times larger than the value for the stack of 3.6 nm aBN + 5 nm HfO2.(k of aBN is 3.9)  .Therefore, it is necessary to define the differences compared to reported aBN structures.In this aspect, I believe that this work may be considered for publication after correcting somewhat misleading statements.
1. Why does aBN show a relatively high dielectric constant (k)?They have higher k than crystalline BN (~3.2).For example, if their deposition temperature exceeds 300 o C, does k value decrease?
Amorphous and crystalline hexagonal boron nitride (hBN) both are reported to exhibit a range of dielectric constants depending on film characteristics and processing methods.In our study, we determined that deposition temperature plays a significant role in the O concentration of the aBN, which consequently impacts the determined aBN dielectric constant.Specifically, as O concentration increases with lower deposition temperature, the dielectric constant increases.The higher κ observed for 65 o C aBN in this study is markedly different from other reported κ of aBN, which are from near stoichiometric aBN, and therefore have minimized dipole contribution from polar B-O components.Based on these findings, we also expect that higher deposition temperatures, such as 300 o C, may further decrease the dielectric constant due to O concentration reduction in aBN.We also note that in this study, temperatures above 250 o C have not been explored in depth due to considerations of aBN as an interface dielectric for 2D applications and recrystallization that might occur at 300 o C. To minimize the 2D material structural damage, aBN is performed at or below the temperature of 250 o C.
2. In Figure 1b, they measured the Raman spectrum of aBN on a Si substrate to confirm a non-crystalline BN structure.This reviewer recommends adding the aBN Raman spectrum measured on the SiO2 substrate because detecting the E2g signal of thin crystalline BN is difficult on the Si substrate.Furthermore, many characterizations of aBN are conducted on SiO2 in this study.The oxide layer can affect the crystallinity of BN during deposition.3. To clarify the thickness of the aBN film, the authors have to show the height profiles of their aBN films as a function of deposition temperature using atomic force microscopy.
We thank the reviewer for the suggestion.We note that while atomic force microscopy (AFM) is generally used for the thickness of the layered hBN phase, it is less ideal for the aBN film.In the case of CVD-grown hBN film or exfoliated hBN flakes, AFM scan can be performed at the flake-substrate boundary to determine the hBN thickness.However, since the phase of the ALD-BN is amorphous, and the film is uniformly coated and extends to the edge of the substrate, no film-substrate boundary is present for the AFM thickness characterization.We have provided the thickness of aBN as a function of deposition temperature as determined by spectroscopic ellipsometry (SE) in Supplementary Figure 3a.
In addition, we prepared cross-sectional samples and used HAADF-STEM and SEM to characterize their height profiles and crystallinity.A set of cross-sectional images for ~ 2, 8, and 11 nm ALD-BN films is provided (Figure R3.2).While doing AFM on such uniformly coated films usually requires us to scratch

(
Q1) Even if an aBN sample grown at 65 degrees undergoes subsequent post-treatment at 200 degrees in the chamber, will it have oxygen contents like the as-grown sample without heat treatment?Our experimental results from the hybrid ALD aBN film stack suggest that the 200 o C aBN ALD transforms the 65 o C aBN component film via two proposed mechanisms: 1) increased extent of reaction between B-Cl and N-H components in the 65 o C aBN film through the elevated temperature and 2) increased extent of reaction with incoming precursors.Both mechanisms likely play a role in the transformation of 65 o C film during the hybrid ALD process, and suggest that a post-treatment of 200 o C thermal annealing will further enhance the unreacted B-Cl and N-H towards stoichiometric aBN.Thus, although post-annealing has not been tested, we expect the post-treatment at 200 o C to result in lower overall O content.
This is within the typical observed deviation range of XPS analysis of the aBN films.The XPS spectra of the two aBN films (pure and hybrid) and their fitted components do not indicate a significant shift or change in intensity (Figure R1.1).

Figure R1. 1 :
Figure R1.1:B 1s XPS spectra and component fits for pure 200 o C and hybrid (65 o C followed by 200 o C) aBN films.

(
Q7) What is the minimum thickness of aBN before HfO2 deposition for MoS2 device fabrication?40 ALD cycles is the minimum cycle number of low-temperature (65 o C) interfacial layer required for forming a continuous higher-temperature (200 -250 o C) aBN film on MoS2 surface.The corresponding minimum thickness of this interfacial layer is about 2.5 nm, such as the ones we used for the quantum well structures made of aBN/MoS2 stacks.Figure R1.2 is a close-view cross-sectional HAADF image that shows ~ 2.5 nm aBN thickness of an example from this recipe.The aBN before HfO2 deposition for the top-gated MoS2 device fabrication was slightly thicker, 3.6 nm, because we wanted to minimize dielectric pinhole formation and or pinhole density on the top MoS2 device surface.

Figure R1. 2 :
Figure R1.2:A HAADF image of an Au-encapsulated aBN film deposited on silica by our ALD.

Figure R2. 2 .
Figure R2.2.a) Representative SEM image of a ML MoS2 MOSCAP.b) C-V characteristics of a ML MoS2 MOSCAP with an area of 150 um 2 and a gate dielectric stack of 3.6nm aBN+5nm HfO2.c) Areadependent capacitance plot.

5 .
Providing additional experimental evidence for the abnormal k values of the aBN grown at 65 degrees Celsius would be beneficial.At a deposition temperature of 65 o C, the aBN film is comprised of a high concentration of O based on XPS quantification using the B-O component (Figure R2.4a).In addition to B-O components, the 65 o C aBN also exhibits N-H components in the N 1s spectra (Figure R2.4b), which is indicative of unreacted N-H because of insufficient thermal energy for a complete reaction.We hypothesize that the anomalous dielectric constant is attributed to the observed higher incorporation of impurities at 65 o C.

Figure R3. 1 :
Figure R3.1:Raman spectra of (a) 10 nm aBN deposited at 225 o C on SiO2 (blue) and bare SiO2 substrate reference (black) and (b) 30 nm aBN deposited at 200 o C on Si.